Venous aneurysms are rare and usually develop following trauma. This report describes an aneurysm of the internal jugular vein that was associated with dysplasia of the cervical arteries, in a patient with neurofibromatosis type 1 (NF1). The finding of neurofibromatous tissue in the wall of the aneurysm as well as in small veins, suggested that the venous aneurysm was caused by the neurofibromatous invasion. No features of the dysplasia encountered in arteries of NF1 lesions were observed in the aneurysm or veins. During and after surgical excision of the aneurysm, the patient developed massive hemorrhages that required two reexplorations and evacuations of cervical hematomas. During surgeries, the bleeding was difficult to control because of excessive friability of the blood vessels. We suggest that the increased vascular fragility in this patient was caused by the NF1-associated arterial dysplasia and by the neurofibromatous venous invasion. Despite the vascular invasion by tumor, there is no evidence of malignancy or malignant transformation in this NF1 patient after a 10-year follow-up.